I don't get it. You could have probably maintained a Debian Sarge install and upgraded it all the way through to Bookworm. I'm kind of surprised they don't provide an upgrade path in place for Raspian when Debian can manage it.
"This time, because the changes to the underlying architecture are so significant, we are not suggesting any procedure for upgrading a Bullseye image to Bookworm; any attempt to do this will almost certainly end up with a non-booting desktop and data loss. The only way to get Bookworm is either to create an SD card using Raspberry Pi Imager, or to download and flash a Bookworm image from here with your tool of choice."
It would be great if they provided more details. Are the issues specific to desktop usage? And to work around it, is it enough to start with a fresh home directory?